The term “phototherapy” relates to the therapeutic use of light. Various light therapies (e.g., including low level light therapy (LLLT) and photodynamic therapy (PDT)) have been publicly reported or claimed to provide various health related medical benefits—including, but not limited to: promoting hair growth; treatment of skin or tissue inflammation; promoting tissue or skin healing or rejuvenation; enhancing wound healing; pain management; reduction of wrinkles, scars, stretch marks, varicose veins, and spider veins; treating cardiovascular disease; treating erectile dysfunction; treating microbial infections; treating hyperbilirubinemia; and treating various oncological and non-oncological diseases or disorders.
Various mechanisms by which phototherapy has been suggested to provide therapeutic benefits include: increasing circulation (e.g., by increasing formation of new capillaries); stimulating the production of collagen; stimulating the release of adenosine triphosphate (ATP); enhancing porphyrin production; reducing excitability of nervous system tissues; modulating fibroblast activity; increasing phagocytosis; inducing thermal effects; stimulating tissue granulation and connective tissue projections; reducing inflammation; and stimulating acetylcholine release.
Phototherapy has also been suggested to stimulate cells to generate nitric oxide. Various biological functions attributed to nitric oxide include roles as signaling messenger, cytotoxin, antiapoptotic agent, antioxidant, and regulator of microcirculation. Nitric oxide is recognized to relax vascular smooth muscles, dilate blood vessels, inhibit aggregation of platelets, and modulate T cell-mediate immune response.
Nitric oxide is produced by multiple cell types in tissue, and is formed by the conversion of the amino acid L-arginine to L-citrulline and nitric oxide, mediated by the enzymatic action of nitric oxide synthases (NOSs). NOS is a NADPH-dependent enzyme that catalyzes the following reaction:L-arginine+3/2 NADPH+H++2O2⇄citrulline+nitric oxide+3/2 NADP+
In mammals, three distinct genes encode NOS isozymes: neuronal (nNOS or NOS-I), cytokine-inducible (iNOS or NOS-II), and endothelial (eNOS or NOS-III). iNOS and nNOS are soluble and found predominantly in the cytosol, while eNOS is membrane associated. Many cells in mammals synthesize iNOS in response to inflammatory conditions.
Skin has been documented to upregulate inducible nitric oxide synthase expression and subsequent production of nitric oxide in response to irradiation stress. Nitric oxide serves a predominantly anti-oxidant role in the high levels generated in response to radiation.
Nitric oxide is a free radical capable of diffusing across membranes and into various tissues; however, it is very reactive, with a half-life of only a few seconds. Due to its unstable nature, nitric oxide rapidly reacts with other molecules to form more stable products. For example, in the blood, nitric oxide rapidly oxidizes to nitrite, and is then further oxidized with oxyhaemoglobin to produce nitrate. Nitric oxide also reacts directly with oxyhaemoglobin to produce methaemoglobin and nitrate. Nitric oxide is also endogenously stored on a variety of nitrosated biochemical structures including nitrosoglutathione (GSNO), nitrosoalbumin, nitrosohemoglobin, and a large number of nitrosocysteine residues on other critical blood/tissue proteins. The term “nitroso” is defined as a nitrosated compound (nitrosothiols (RSNO) or nitrosamines (RNNO)), via either S- or N-nitrosation. Examples of nitrosated compounds include GSNO, nitrosoalbumin, nitrosohemoglobin, and proteins with nitrosated cysteine residue. Metal nitrosyl (M-NO) complexes are another endogenous store of circulating nitric oxide, most commonly found as ferrous nitrosyl complexes in the body; however, metal nitrosyl complexes are not restricted to complexes with iron-containing metal centers, since nitrosation also occurs at heme groups and copper centers. Examples of metal nitrosyl complexes include cytochrome c oxidase (CCO-NO) (exhibiting 2 heme and 2 copper binding sites), cytochrome c (exhibiting heme center binding), and nitrosylhemoglobin (exhibiting heme center binding for hemoglobin and methemoglobin), embodying endogenous stores of nitric oxide.
FIG. 1 is a reaction sequence showing photoactivated production of nitric oxide catalyzed by iNOS, followed by binding of nitric oxide to CCO.
When nitric oxide is auto-oxidized into nitrosative intermediates, the nitric oxide is bound covalently in the body (in a “bound” state). Thus, conventional efforts to produce nitric oxide in tissue may have a limited therapeutic effect, since nitric oxide in its “gaseous” state is short-lived, and cells being stimulated to produce nitric oxide may become depleted of NADPH or L-Arginine to sustain nitric oxide production.